Various membrane processes may generally be applied for example, in water treatment and industrial treatment applications. The cost effectiveness of these processes depends, among other factors, on the combination of the membrane properties, such as flux, separation characteristics, performance stability, and fouling resistance as well as the ability to be cleaned when fouled.
For many applications properties such as chemical resistance, high oxidant or aqueous halogen solution (hypochlorite) resistance, as well as flux and rejection, are considerably important. However, it appears that this combination of properties is difficult to achieve. For water treatment and desalination applications, for example, to-date commonly used membranes are reverse osmosis (RO) and/or nanofiltration (NF) membranes based on polyamide composites. These membranes may generally be considered to have good flux and rejection characteristics, but from the stability and chemical resistance point of view, there is still a need for improvement. Other RO and NF membranes, such as polyvinyl alcohol and sulfonated engineering plastics membranes, may generally be considered to have better chemical resistance but suffer from inferior flux/selectivity characteristics compared to the polyamide composites membranes. In addition, they do not have high oxidant resistance or specific resistance to widely used hypochlorite disinfectants. Therefore, the development and manufacture of membranes, such as RO and NF membranes, having the flux/selectivity/rejection properties similar to (or better than) that of the polyamide membranes, but with higher chemical resistance, particularly oxidant and/or halogenation resistance which occurs on exposure to aqueous halogen solution (hypochlorite), is of great importance and may significantly lower the cost of water treatment.
Membranes, such as RO membranes, with higher salt rejections are also highly important, for example, for seawater desalination. In desalination of water streams, especially seawater and brackish water, high rejections without loss of flux would decrease the cost of the final water product. In addition, there are many applications in water purifications where one would like to remove boron or nitrates. Existing membranes, such as RO and NF membranes, still do not have sufficiently high and long-term rejections to these components, and any membrane or modification that can improve the rejection of RO and NF membranes to these solutes is of great importance.
There are ionic polymers, such as polyelectrolytes and ionomer materials, which offer good potential for making membranes, for example, highly stable selective RO and NF membranes, if they can be prepared in a given morphology and fixed into this morphology by covalent crosslinking. In general, the problem with non-crosslinked thin selective layers, especially for RO and NF membranes based on ionomers and polyelectrolytes, is the continuous swelling and change in performance over time and/or changes in the ionic strength of the solution. However, chemically stable covalent crosslinking in chemically stable ionomer materials is difficult to achieve, especially when improved oxidant and halogenation resistance is desirable.
Patent application WO2007135689 discloses membranes and/or films (for example, from a polymer or blend(s) of polymers) having a given morphology which is stabilized, for example against swelling and chemical degradation, by covalent crosslinking and optionally, in addition, by hydrophobization. The membrane made above was placed in the following solutions to check stability: pH 11-12 for 14 days at room temperature, pH 1-2 for 14 days at room temperature and 20-30 ppm NaOCl pH 10 for 14 days at room-temperature. In all solutions the membrane performance remained stable.
U.S. Pat. No. 4,990,252 discloses a novel thin film composite or coated membrane suitable for reverse osmosis, ultrafiltration and microfiltration applications, and having a porous polymeric substrate with one or more microporous layers to which a thin film or coating comprising a sulfonated polyarylether is attached substantively to provide an oxidatively stable, thin hydrophilic film or coating layer, and a method for manufacturing and using the same. Some of the examples included the examination of porous polysulfone substrate coated with a solution of sulfonated polyethersulfone.
Patent application US 20070163951 discloses a method of making a chlorine tolerant hydrophilic-hydrophobic copolymer desalination membrane. The membrane is made by forming a hydrophilic-hydrophobic random copolymer having one or more hydrophilic monomers and one or more hydrophobic monomers and forming the hydrophilic-hydrophobic random copolymer into a hydrophilic-hydrophobic copolymer desalination membrane. The hydrophilic monomers include a sulfonated polyarylsulfone monomer and a second monomer, while the hydrophobic monomers include a non-sulfonated third monomer and a fourth monomer. The sulfonated polyarylsulfone monomer introduces the sulfonate into the hydrophilic-hydrophobic random copolymer and provides a chlorine tolerant hydrophilic-hydrophobic copolymer desalination membrane.
Based on the similarities in chemical compositions as described in Patent application WO2007135689 and U.S. Pat. No. 4,990,252 these membranes are not expect to maintain their flux under the conditions 500-1000 ppm NaOCl pH 9 and 20° C.
Patent application US 20020045085 discloses composite solid polymer electrolyte membranes (SPEMs) which include a porous polymer substrate interpenetrated with an ion-conducting material. The stability of the ion-conducting polymer may be enhanced by several post-processing steps, including chlorination/bromination of the ion-conducting polymer backbone, thereby reducing degradation sites.
The post treatment is not a practical approach for many membranes and coatings. In addition, when the post halogenation step is carried out as described in the above application, it is difficult to control the balance between the crosslinking and hydrophobicity/hydrophilicity, which is required in order to achieve stability, good flux and selectivity properties.
Thus, there is an unmet need in the art for resistant membranes characterized by stable covalent crosslinks, with high resistance to oxidants and halogenation reactions occurring, for example, in aqueous halogen solution (hypochlorite), that also maintain good flux/rejection/selectivity properties.